Conventionally, fiber-optic communication networks are experiencing rapidly increasing growth of capacity. This capacity growth is reflected by individual channel data rate scaling from 10 Gbps, to 40 Gbps, to currently developing 100 Gbps, and to future projections of 1000 Gbps channels and beyond. The capacity growth is also reflected by a desire to increase aggregate fiber carrying capacity by increasing total channel count. The desired capacity growth can be addressed by several techniques. First, the bandwidth of optical amplifiers can be increased to allow wider spectral range to be used for signal transmission. This approach is viable for new network installations, where new amplifiers can be deployed. This approach is not applicable to a large base of installed networks, and would also require a development of other associated wide spectral range components, such as lasers, optical filters, and dispersion compensation modules. Another approach is to use multi-bit per symbol modulation constellations, such as M-ary quadrature amplitude modulation (M-QAM). Increasing the constellation size M increases the information transmission capacity, while keeping the signal bandwidth constant. Unfortunately, this comes at a very substantial penalty of increased noise susceptibility, and correspondingly reduced optical unregenerated reach.
Another approach is to use smaller spacing between wavelength division multiplexed (WDM) channels. Currently, per ITU standard specification, WDM channels are placed with 50 GHz spacing. Furthermore, channels are typically combined and separated using optical filters. Thus, individual channels are filtered on the transmitter side such that the overlap between adjacent channel frequency content is made negligible. Similarly, receiver side optical filtering is used to accept the frequency content of a single channel, while effectively rejecting all adjacent channels. This approach results in a substantial waste of valuable spectrum to accommodate channel isolation and filter roll-off skirts. An attractive approach for increasing spectral efficiency is to use a set of subcarriers, each modulated by data of identical rates and locked precisely to that data rate. This approach is widely used in communications, and is generally known as Orthogonal Frequency-Division Multiplexing (OFDM) in wireless or Discrete Multi-Tone (DMT) in Digital Subscriber Loop (DSL) applications.
Previous attempts to satisfy some of the above requirements for fiber-optic communication are enumerated below, with associated benefits and drawbacks. Ellis et al. in “Towards 1TbE using Coherent WDM,” Opto-Electronics and Communications Conference, 2008 and the 2008 Australian Conference on Optical Fibre Technology. OECC/ACOFT 2008. 7-10 July 2008, page(s): 1-4 disclose a coherent WDM approach using On-Off Keyed modulation and putting subcarriers onto a grid precisely locked to the data rate. The receiver uses optical filtering to select individual subcarriers and subsequent direct detection for conversion to electrical domain. Advantages of this approach include a relatively simple transmitter and receiver, with minimal processing and an optoelectronic component bandwidth requirement of only a single subcarrier rate. However, drawbacks are substantial and include low chromatic and polarization mode dispersion tolerance, inability to scale to phase-based or multi-symbol modulation formats, and poor amplified spontaneous emission (ASE) noise tolerance.
Coherent Optical OFDM is essentially a direct application of wireless OFDM principles to the optical domain (see, e.g., “Coherent optical OFDM: theory and design,” W. Shieh, H. Bao, and Y. Tang, Optics Express, vol. 16, no 2, January 2008, pp. 841-859). “Virtual” subcarriers with superimposed data modulation are generated in digital electronics via Inverse Fast Fourier Transform (FFT) operation on the transmit side. Original data is recovered via a complementary FFT operation on the receive side. Advantages of this approach are very high tolerance to chromatic and polarization mode dispersion, scalability to arbitrary modulation constellations, and high tolerance to ASE noise. However, disadvantages are the requirement for sophisticated digital signal processing on the transmitter (FFT) and receiver (IFFT) operating on complete channel data, and a requirement for adding redundant cyclic prefix data. The requirement for optoelectronic component bandwidth to cover a complete channel is detrimental. Further, subcarriers within the OFDM channel are sufficiently low frequency such that complex phase recovery techniques are required. As is common in classical OFDM and DMT the transmitted signal exhibits a near Gaussian distribution which demands higher Digital-Analog Conversion (DAC) and Analog-Digital Conversion (ADC) dynamic range (i.e. more bits of precision) and/or the use of additional signal processing to mitigate the high peak to average signal power.
Subband multiplexed Coherent optical OFDM extends the above concepts by stacking several OFDM channels very close together to form a quasi-continuous spectrum. For example, these are disclosed in “Coherent optical OFDM transmission up to 1 Tb/s per channel,” Y. Tang and W. Shieh, J. Lightwave Techn., vol. 27, no. 16, August 2009, pp. 3511-3517, and “Optical comb and filter bank (de)mux enabling 1 Tb/s orthogonal sub-band multiplexed CO-OFDM free of ADC/DAC limits,” M. Nazarathy, D. M. Marom, W. Shieh, European Conference on Optical Communications (ECOC) 2009, paper P3.12, September 2009. Advantages are the same as Optical OFDM, with an ability to extend complete channel coverage to arbitrary total capacity (Assuming synchronous data is provided). Disadvantages are similar to optical OFDM, dominated by signal processing complexity. It is unlikely that such an approach will be practical and realizable considering the associated electro-optic power consumption. Further, sharp roll-off optical filters may be required in some implementations for sub-band separation. Binary Phase Shift Keying (BPSK) channels optically combined have been shown to be a possibility with direct detection receiver (e.g., “Over 100 Gb/s electro-optically multiplexed OFDM for high-capacity optical transport network,” T. Kobayashi, et al, J. Lightwave Techn., vol. 27, no. 16, August 2009, pp. 3714-3720) and Coherent Detection (e.g., “Orthogonal wavelength-division multiplexing using coherent detection,” G. Goldfarb, et al, IEEE Photonics Techn. Lett., vol. 19, no. 24, December 2007, pp. 2015-2017). Advantages are a partitioning of complexity between optical and electrical processing, such that overall complexity and power consumption can be reduced. Disadvantage stems from the fact that proper operation still requires optoelectronic bandwidth on the order of the total spectrum encompassing the complete channel, which is substantially beyond the state of the art assuming a 1000 Gbps channel. Referenced papers use lower bandwidth components, and correspondingly show a very substantial and detrimental performance penalty of several dB.
As discussed above, current state-of-the-art has several shortcomings, with each proposed implementation suffering from at least one of the following. First, complex digital signal processing is required at both transmitter and receiver (i.e. for OFDM) to compress signal spectrum and provide dispersion tolerance. The Application Specific Integrated Circuit (ASIC) size and power consumption associated with this processing is prohibitive for scaling to 1000 Gbps (1 Tb/s) transceiver. Also, for OFDM, the DAC and ADC resolution required continues to increase well beyond the current state of the art when the necessary sample rates are considered. Second, maintaining subcarrier orthogonality requires electro-optic component bandwidths that cover substantially all of a channel spectrum, which may be approximately 300 GHz wide. Components with such bandwidth are not expected to be available for many years to come. Third, performance loss associated with sub-optimal component performance or compromised digital signal processing results in a prohibitively low unregenerated link budget in fiber-optic networks.